During open heart surgery the blood of the patient is bypassed to an extra corporeal blood circuit. The circuit commonly includes a support system which supplies the pumping function of the heart and the oxygenation function of the lungs. This effectively isolates the heart and lungs enabling the surgeon to make the necessary repairs to the heart and/or lungs. Both venous blood and cardiotomy blood from the surgical site may be removed and circulated through the extra corporeal circuit.
Blood filters are typically included both upstream and downstream from the oxygenator which may incorporate a heat exchanger. Upstream blood filters include venous and cardiotomy filters through which the blood may be filtered prior to entering the oxygenator and which are used to remove particulate, especially from the surgical site, and bubbles from the blood. Although these upstream filters are effective it is possible that some emboli may pass through or be generated in the oxygenator and/or heat exchanger. These emboli may be in suspension in the oxygenated blood which is to be returned to the patient. The embolic material can be either particulate, such as platelet or white cell aggregates, or gaseous, such as small or large gas bubbles. Therefore, an arterial line filter which is located downstream of the oxygenator is critically important in trapping and removing any remaining emboli before the blood is provided to the patient.
A typical conventional arterial blood filter is illustrated in FIG. 1. The filter includes a blood inlet located at the top of the filter. In this case the inlet is located in a tangential position. A radially pleated tubular filter is disposed within a cylindrical housing and is covered by a cortically shaped cap. The arrow shows the path of blood as it flows through the filter. After entering through the tangential inlet the blood travels in a circular path over and around the cap and down between the outer wall of the housing and the pleated filter. The blood eventually passes through the filter and is discharged through a blood outlet at the bottom of the device. A gas port is located at the top of the device through which air bubbles or gaseous emboli may be vented.
Although conventional arterial blood filters are effective in achieving significant reductions in emboli they share several common problems. First, they include significant horizontal surface area in the blood flow path. Bubbles tend to attach themselves to these horizontal surfaces increasing the difficulty of priming and, therefore, making the filters more difficult to use. Second in many radially pleated designs, blood flows down through a small annular gap between the filter element and the housing. At a given flow rate, the smaller this area is, the higher is the downward blood velocity. Higher blood velocities entrain smaller air bubbles which subsequently get trapped in the filter element or get broken up into smaller bubbles and pass through the filter. It would be desirable to have as large an area as possible available for downward flow to reduce downward entrainment of small air bubbles. Third, in many conventional arterial filters the priming volume is not utilized efficiently. Generally, the bigger the prime volume of a filter, the longer the time available for air bubbles to separate because of their buoyancy. However, there is also a desire to minimize prime volume in order to avoid unnecessary use of blood and blood products. The best balance is achieved by ensuring that as much of the prime volume as possible is upstream of the filter screen, because this is the region in which air can separate by buoyancy. Fourth, in most radially pleated designs the frontal area for flow downstream of the filter element is substantially reduced from the frontal area upstream of the filter element. Thus the blood velocity significantly increases after it passes through the filter element in conventional designs. This higher velocity tends to entrain bubbles which have passed through the filter element and carries them toward the outlet port where the bubbles may be conducted to the patent. It is desirable to maintain the frontal area for flow adjacent to the downstream side of the filter element as large as possible, or at least as close as feasible to the frontal area upstream of the filter element. Fifth, radially pleated designs incorporate a shallow annular or disk shaped potting cup to mount the top and bottom of the filter element. The filter element end is immersed in the liquid potting compound and held in position until the compound solidifies. Since the cup is rarely completely filled with the potting compound, a concave surface is formed by the potting compound and the lip of the cup. Concave surfaces can also be created by the liquid potting compound wicking up the filter element while the compound is hardening from a liquid to a solid. In use bubbles can become trapped under the concave surface of the upper potting cup. When the filter is inverted during priming to remove these trapped bubbles they can rise and be trapped under the concave surface of the bottom potting cup which is now on top. This bubble trapping complicates priming the filter. It is desirable to create a design which does not require potting either the top or bottom of the filter element. Thus, it would be desirable to provide an arterial blood filter which not only has efficient emboli removal capabilities but is also easier to prime and provides more opportunity for buoyancy separation of air than do conventional filters.